Article pubs.acs.org/IECR
On-Line NMR Spectroscopic Reaction Kinetic Study of Urea− Formaldehyde Resin Synthesis Éléonore J. Kibrik,†,∥ Oliver Steinhof,⊥ Günter Scherr,¶ Werner R. Thiel,‡ and Hans Hasse*,§ †
Laboratory of Engineering Thermodynamics (LTD), ‡Department of Chemistry, and §Department of Engineering Thermodynamics, University of Kaiserslautern, Kaiserslautern 67663, Germany ⊥ Institute of Thermodynamics and Thermal Process Engineering (ITT), University of Stuttgart, Stuttgart 70174, Germany ¶ BASF SE, Ludwigshafen 67063, Germany S Supporting Information *
ABSTRACT: Quantitative on-line NMR spectroscopy is used to study the kinetics of the reaction of aqueous formaldehyde and urea. The investigation focuses on the formation of low molecular mass compounds during the methylolation step. The experiments were carried out at overall formaldehyde to urea molar ratios between 1:1 and 4:1, pH values between 6 and 8, and temperatures between 313 and 353 K. The experimental data were used to develop a kinetic model based on the true species concentrations. The model describes the experimental data well and can be used to predict the composition of the reacting mixture of aqueous formaldehyde and urea during the methylolation step as a function of time.
1. INTRODUCTION Quantitative on-line NMR spectroscopy was used in this work to continuously monitor the reaction of aqueous formaldehyde and urea during urea−formaldehyde (UF) resin synthesis. UF resins are common binders in the manufacture of wood products such as particle boards and plywood. They have been manufactured industrially since the early 1930s (IG Farbenindustrie, CIBA, Toledo Synthetic Products, Société Nobel Française),1 but despite this long commercial history, the kinetics of the complex reaction system of the UF resin synthesis still cannot be satisfactorily described. An overview of the available reaction kinetic studies of the UF system in the literature is given in the work of Carvalho et al.2 and in the Supporting Information. On-line NMR spectroscopy is well established for studying reaction kinetics of complex reaction networks.3,4 It enables structure elucidation as well as the acquisition of quantitative data on sample composition in real time over a wide range of conditions without sample preparation. On-line NMR spectroscopy is thus well suited for the analysis of the complex UF reaction system. NMR spectroscopy has been used before for studying various aspects of the UF resin synthesis (see the Supporting Information). We have recently presented a comprehensive study on the quantitative and qualitative analysis of the UF reaction system.5 That work lays the methodological foundations for the present study. In related work, the formation of ether-bridged condensation products in UF systems was investigated.6,7 In the present work, a kinetic model of the reacting mixtures of aqueous formaldehyde and urea was developed based on comprehensive NMR spectroscopic data. It focuses on the formation of low molecular mass compounds; the formation of methylene-bridged condensation products is only considered to a limited extent. The model is based on a true species concentration, with monomeric formaldehyde as the key reactive species. The formation of formaldehyde oligomers © 2014 American Chemical Society
with compounds containing hydroxyl groups is accounted for explicitly. The model parameters were adjusted to on-line NMR spectroscopic data taken at overall formaldehyde to urea (FA:U) molar ratios (MRFA:U) in the range of 1:1−4:1, pH values of 6−8, and temperatures of 313−353 K, which were obtained using an aqueous formaldehyde solution with an overall concentration of formaldehyde of 0.3 and 0.5 g g−1 as starting materials. The range of the studied MRFA:Us is more broad than that of the MRFA:Us of the first UF resin synthesis step (methylolation step, typical MRFA:U between 1.8:1.0− 2.5:1.0) in order to understand the influence of the molar ratios at the more extreme ends of the variation. The studied ranges of pH values and temperatures were restricted for practical reasons. At high pH (>8), the broadening of peaks due to the increased exchange rate of NH protons prevents meaningful peak evaluation. At low pH ( 1) MRFA:U = overall formaldehyde to urea molar ratio n = number of CH2O groups in poly(oxymethy1ene) glycols or methylolurea hemiformals nji = number of protons in the group j of the species i N = number of measurements data in all experiments NM,i = number of measurement data for the variable i NV = number of variables of the experiment NMR = nuclear magnetic resonance o.d. = outer diameter pH = pH value p. a. = pro analysi R = universal gas constant t = time T = temperature UF = urea−formaldehyde W = water x̃FA+W = overall concentration of formaldehyde in aqueous FA formaldehyde solution Zji = number of protons giving rise to the signal Aji* zij = calculated value j from variable i Greek Letters
Δrh = enthalpy of reaction Δrs = entropy of reaction ζi = pseudoconcentration of i, referring to the overall urea concentration ηi = amount of formaldehyde bound in each species i, referring to the overall formaldehyde concentration σ2ij = variance of measurement j of variable i σ2ij = assumed variance of measurement j of variable i χ2 = chi squared Subscripts
free = overall formaldehyde concentration bound = formaldehyde bound in urea−formaldehyde compounds ref = reference Superscripts 0
■
= start
REFERENCES
(1) Meyer, B. Urea-Formaldeyde Resins; Addison-Wesley Publishing Company: Boston, MA, 1979. (2) Carvalho, L. M.; Costa, M. R.; Costa, C. A. A Very Simple Empirical Kinetic Model of the Acid-Catalyzed Cure of Urea− Formaldehyde Resins. J. Appl. Polym. Sci. 2006, 102, 5977−5987. (3) Maiwald, M.; Fischer, H. H.; Kim, Y. K.; Hasse, H. Quantitative On-Line High-Resolution NMR Spectroscopy in Process Engineering Applications. Anal. Bioanal. Chem. 2003, 375, 1111−1115. (4) Maiwald, M.; Fischer, H. H.; Kim, Y. K.; Albert, K.; Hasse, H. Quantitative High-Resolution On-Line NMR Spectroscopy in Reaction and Process Monitoring. J. Magn. Reson. 2004, 166, 135−146. 12612
dx.doi.org/10.1021/ie5001746 | Ind. Eng. Chem. Res. 2014, 53, 12602−12613
Industrial & Engineering Chemistry Research
Article
(LTD), University of Kaiserslautern: Kaiserslautern, Germany, 2014, ISBN 978-3-944433-09-7. (26) de Jong, J. I.; de Jonge, J. Kinetics of the Formation of Methylene Linkages in Solutions of Urea and Formaldehyde. Recl. Trav. Chim. Pays-Bas 1953, 72, 139−156. (27) Glutz, B. R.; Zollinger, H. General Acid−Base Catalysis of Monomethylol Formation from Urea and Formaldehyde in Water. Helv. Chim. Acta 1969, 52, 1976−84.
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dx.doi.org/10.1021/ie5001746 | Ind. Eng. Chem. Res. 2014, 53, 12602−12613
